A gas turbine includes a rotor that rotates around an axis and a casing that covers this rotor. The rotor has a rotor shaft and a plurality of blades mounted on this rotor shaft. The blade has a blade body that forms an airfoil, a platform that spreads from an end in a blade height direction of the blade both in a direction roughly perpendicular to the blade height direction, and a shaft-mounted part that extends from the platform toward the opposite side from the blade body.
The blades of a gas turbine are exposed tea high-temperature combustion gas. Therefore, the blades are commonly cooled with air etc.
For example, the blade described in Patent Literature 1 below has various cooling passages through which cooling air passes. Specifically, blade passages through which cooling air flows are formed inside the blade body, the platform, and the shaft-mounted part so as to extend in a blade height direction. The platform has a gas path surface that faces the blade height direction and comes in contact with combustion gas, and a shaft-side surface that is located back-to-back with the gas path surface. This platform further has a platform passage which extends between the gas path surface and the shaft-side surface in a blade thickness direction and through which cooling air flows, and a turndown extension portion that extends from an end of the platform passage on the blade passage side in a direction away from the gas path surface. A communication passage that leads from an outer surface at a corner between the shaft-side surface of the platform and an outer surface of the shaft-mounted part through the turndown extension portion to the blade passage is formed inside the platform and the shaft-mounted part. An opening of this communication passage in the outer surface at the corner is blocked with a plug etc.
This blade is basically manufactured by casting. The blade passage, the platform passage, and the turndown extension portion are formed by a casting process using cores that have external shapes matching their respective shapes. In an intermediate product of the blade formed by casting, the platform passage and the turndown extension portion communicate with each other, while the turndown extension portion and the blade passage do not communicate with each other. The communication passage is formed after this intermediate product is formed. Specifically, a through-hole that extends from the outer surface at the corner of the intermediate product through the turndown extension portion to the blade passage is formed by machining. This through-hole, i.e., the communication passage, bisects the turndown extension portion into a part on the gas path surface side and a part on the shaft-side surface side.